Q. If an author insists on using a widely attributed quote whose source cannot be confirmed, how should I cite it? Should I cite it at all? Or should I simply note in the running text that the quote is “widely attributed to such and such”?
A. Your last idea is the best one. In general, if the source of a quotation can’t be confirmed, this fact should be stated in the text that introduces it instead of being relegated to a note, where readers might miss it. The phrase “widely attributed” should make it clear that the attribution isn’t definitive; however, if the author can provide one or more sources that back up this claim, those could be (and in an academic work should be) provided in a note.